(#1785) Update test_framework.py to support version compatibility tests

[cpacia]

The current way of running tests is:

python3 test_name.py -b /path/to/obgo-binary -d /path/to/bitcoind

This way still works, but a new option is available:

python3 test_name.py -i /path/to/obgo-binary-v1 /path/to/obgo-binary-v2 -d /path/to/bitcoind

Where i is a list of string paths for each ob-go binary that is used. The binaries can be different versions. The number of binaries in the list must be equal to the number of nodes created by the test and the index of the binary string must map to the node (buyer, vendor, moderator) that you want it to map to.

This PR is technically incomplete as it only allows the use of multiple binaries, however all the test data is currently formatted using the current version and not prior versions. Getting the old version shouldn't be difficult as it's just copying and pasting the test data from the old version. However, each test will need to be updated to decide which version of the data file it needs to load and post to the server.

I've added another flag -v that can be used to specify the versions which map to each node. For example:

`-v v1 v2 v2`

Means that node 0 is a v1 node and nodes 2 and 3 are v2 nodes. Each test would need to do something like:

alice = self.nodes[1]

file_path = "testdata/listing.json"
if self.versions is not None and self.versions[1] is "v1":
    file_path = "testdata/listing_old.json"
with open(file_path) as listing_file:
    listing_json = json.load(listing_file, object_pairs_hook=OrderedDict)

[coveralls]

Coverage remained the same at 41.662% when pulling 13d28b1 on compatibilitytest into 09c1138 on ethereum-master.

[hoffmabc]

@cpacia are there any tests being run against different versions in this PR so far?

[cpacia]

v4 tests:

• cancel_direct_offline
• chat
• chat_offline
• complete_direct_online
• complete_disputed
• complete_moderated_online
• complete_moderated_with_timeout
• dispute_close_buyer
• dispute_close_vendor
• dispute_close_split
• escrow_release_after_timeout
• fulfill_direct_online
• listings
• manage_crypto_listings
• market_price_modifier.py
• out_of_inventory
• patch_profile
• purchase_crypto_listing
• purchase_digital
• purchase_direct_online
• purchase_direct_offline
• purchase_moderated_offline
• purchase_moderated_online
• purchase_offline_error
• receive_coins
• refund_direct
• refund_moderated
• reject_direct_offline
• reject_moderated_offline
• smtp_notifications
• upload_listing